This invention relates to an image reading section in an image reading device which is employed as image reading means in facsimiles, image scanners, etc., and more particularly to a charge detecting method of detecting the charges which photo-electric conversion elements produce in correspondence to the density data of minute regions in an original image, and a charge detecting circuit for practicing the method.
An image reading device set in close contact with an original in order to read the image of the latter comprises: a photo-electric conversion element array made up of a plurality of photo-electric conversion elements arranged in a line; and a drive IC for driving the photo-electric conversion element array. The drive IC includes switches which are adapted to select the photo-electric conversion elements forming the photo-electric conversion element array one after another to apply the charges generated in the photo-electric conversion elements to one output line in a time-sequential manner.
The photo-electric conversion element array has a light receiving section made up of a plurality of photo-electric conversion elements. Each element is formed by arranging a metal electrode and a transparent conductive film on both sides of an amorphous silicon (a-Si) layer, so as to detect optical charges formed by light reflected from an original image.
A simple equivalent circuit for one bit of the image reading device is as shown in FIG. 7. The circuit operates as follows: When a light beam reflected from an original and including data on the density of a small region of the original image is applied to a photo-diode PD, an optical current Ip flows in the photo-diode PD to produce an optical charge therein. The charge thus formed is stored in a capacitor Cp formed by the light receiving element and a capacitor CL formed by wiring (hereinafter referred to as "a wiring capacitor CL", when applicable), so that the voltage Va of an input line of an amplifier A is increased. The amplifier A detects the voltage Va with high input impedance. The output of the amplifier A is applied by means of an analog switch SW to an output line T.sub.out for every bit, to form a time-series signal. Thereafter, the amplifier A is reset; more specifically, the input line of the amplifier A is grounded through a reset switch RS. Therefore, as the wiring capacitor CL decreases, the voltage Va of the input line of the amplifier A is increased, and accordingly the signal detection accuracy is increased.
The above-described image reading device is disadvantageous in that, since a number of photo-electric conversion elements 70 are driven individually, it is necessary to use a number of driving ICs with the result that the manufacturing cost is increased as much. In order to overcome this difficulty, a matrix drive type image reading device has been proposed in the art which is lower in manufacturing cost with the number of driving ICs decreased.
The matrix drive type image reading device, as shown in FIG. 8, comprises: K photo-electric conversion element groups each consisting of n photo-electric conversion elements 70; and switching elements T.sub.ll -T.sub.kn which are provided for the photo-electric conversion elements 70, respectively. The switching elements T.sub.ll -T.sub.kn are connected to n common signal lines 80. For every block, the switching elements T.sub.ll -T.sub.kn are turned on by gate pulses applied to gate lines Gl-Gk, so that several bits are connected to the common signal lines at the same time, thus being processed in a parallel mode.
For simplification in description, the operation of the image reading device will be described with reference only to the first block. It is assumed that, when the switching elements T.sub.ll -T.sub.ln are turned off, a light beam reflected from an original which includes data on a small region of the original image is applied to the image reading device. In this case, in response to the light beam, optical currents Ip flow, thus producing optical charges. The charges thus produced are stored in the light receiving element capacitors C.sub.Pll -C.sub.Pln and in the overlap capacitors C.sub.GD between the drains and gates of the switching elements. When the switching elements T.sub.ll -T.sub.ln are turned on, the aforementioned charges are distributed to the overlap capacitors C.sub.GS between the sources and gates of the switching elements, the wiring capacitors C.sub.Ll -C.sub.Ln, the light receiving element capacitors C.sub.Pll -C.sub.Pln, and the overlap capacitors C.sub.GD. Therefore, in order to sufficiently transfer the charge to the wiring capacitors C.sub.L, the capacitance must be much larger than the light receiving capacitance C.sub.P, and the overlap capacitances C.sub.GS and C.sub.GD. The changes in potential of the common signal lines 80 due to the charges stored in the wiring capacitors C.sub.Ll -C.sub.Ln are transmitted through amplifiers A.sub.l -A.sub.n to an output line T.sub.out by closing analog switches SW.sub.l -SW.sub.n in a driving IC 81 one after another, so that they are detected in a time-sequential manner. The above-described operation is carried out for every block, so that an image signal is formed for one line of the original.
In the image reading device described with reference to FIG. 7 which employs the potential detecting method, the signal detection accuracy is increased with the decreased capacitance of the wiring capacitor, as was described before; however, it is impossible to decrease the capacitance because of the following reason: In order to allow an optical current to flow in the photo-diode PD, the reverse bias voltage VB across the photo-diode PD must be sufficiently high. If the wiring capacitor CL is small in capacitance, then the reverse bias voltage VB is decreased as the voltage across the wiring capacitor CL increases. If the effective reverse bias voltage VB of the photo-diode PD decreases in this manner, then it becomes impossible to supply the optical current. However, it should be noted that the voltage across the wiring capacitor can be increased as the capacitance of the wiring capacitor decreases as long as the reverse bias voltage VB does not adversely affect the optical current.
In the image reading device of matrix drive type described with reference to FIG. 8, the charges are transferred through the switching elements T.sub.kn to the wiring capacitors C.sub.L and stored therein. Hence, in order to improve the charge transferring efficiency, it is necessary to make C.sub.L much larger than (C.sub.P +C.sub.GD). For this purpose, it is necessary to reduce the voltage developed across the wiring capacitor C.sub.L to a small fraction of that provided on the side of the photo-electric conversion element 70. Hence, the voltage is amplified in the driving IC 81, thus increasing the sensitivity. In this operation, offset noises or random noises occur with the driving IC 81, thus lowering the S/N ratio.